Presently, phosphoric acid is produced by the wet process in which sulfuric acid is used to treat a fine phosphate rock as shown in the following reaction.Ca5F(PO4)3+5H2SO4+H2O→5CaSO4•2H2O↓+3H3PO4+HF  (1)
In this process dihydrated calcium sulfate (also called gypsum or phosphogypsum) is produced as a waste. From Reaction 1, it can be seen that every three moles of phosphoric acid production requires 5 moles of sulfuric acid. A ratio of sulfuric acid to phosphoric acid is 5/3.
The phosphate rock also contains several impurities, such as such as Mg, Al, Si, Fe, Mg, Zn, Mn, Cr, Na K and other metals. These impurities consume sulfuric acid. By considering the acid consumption by these impurities, the ratio of sulfuric acid to phosphoric acid can be 2 or higher.
Reaction 1 shows the reaction between phosphate rock and sulfuric acid. However, even if phosphate rock is very well ground, only the outside surface of the phosphate rock reacts with sulfuric acid. This “surface only” reaction can be attributed to a rapid reaction between sulfuric acid and phosphate rock forming a gypsum film or layer on the surface of the phosphate rock. The inside of the phosphate rock particles remains un-reacted causing an increase in the quantity of gypsum waste as well as causing a reduction in the amount of phosphate recovered.
To address this issue, Reaction 1 is performed as two separate reactions, Reactions 1a and 1b as shown below.2Ca5F(PO4)3+12H3PO4→9 Ca(H2PO4)2+CaF2  (1a)9Ca(H2PO4)2+CaF2+10H2SO4+20H2O→18H3PO4+2HF+10CaSO4•2H2O↓  (1b)
In Reaction 1a, the phosphate rock is first reacted with phosphoric acid to produce monobasic calcium phosphate (monocal) slurry. Then, in Reaction 1b, the monobasic calcium phosphate slurry is reacted with sulfuric acid to produce phosphoric acid and calcium sulfate dihydrate (gypsum). Filtration is used to separate gypsum.
The phosphoric acid required in Reaction 1a is obtained from the recycling of phosphoric acid in the plant. From Reactions 1a and 1b, it can be seen that 12 moles of 18 moles of phosphoric acid produced is recycled or 67% of total phosphoric acid produced needs to be recycled. The remaining 33% of the phosphoric acid can be used to produce ammonium phosphate and other phosphate products. For example, monoammonium phosphate (or MAP) may be produced by reacting ammonia with phosphoric acid according to reaction Q.NH3+H3PO4=>(NH4)H2PO4  (Q)
The reaction condition in Reaction 1 is very acidic due to the addition of sulfuric acid, which causes all the impurities in the phosphate rock to be leached out into solution. The phosphoric acid in solution is therefore impure and has a dark brown color. This impure phosphoric acid is currently used to produce ammonium phosphate fertilizer by reacting ammonia with the impure phosphoric acid (reaction Q). Therefore, the resulting fertilizer grade ammonium phosphate is of low purity. However, as the resulting ammonium phosphate is used as fertilizer, purity of is not of major consequence.